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How Resolution Independence is Working

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I know that Unity3D uses DirectX and OpenGL at backhand for rendering. I am curious about one thing that is Resolution Independence for 2D and 3D is provided by graphical API automatically ? Or Unity3D is providing that feature

Because when i change my game resolution through changes automatically, i just want to know whether this is provided by graphical API ? Or engine is providing this component for graphical API for 2D and 3D

A combination of both. Most graphics APIs, and OpenGL and DirectX are no exception, provide for flexible scaling while allowing pixel operations or at least pixel hinting such that an application can be as pixel-unaware or as pixel-perfect as it needs. Unity continues that tradition such that a typical Unity game might be pixel-unaware for the 3D scene and pixel-perfect for the GUI controls.